1. Field of the Invention
The field of the invention is magnetic resonance imaging systems and, in particular, signal processors associated with such equipment for monitoring bio-potential signals.
2. Background Art
Magnetic resonance imaging ("MRI") has developed as an important tool in diagnostic medicine In an MRI "scan", a body being imaged is held within a uniform magnetic field oriented along a z axis of a Cartesian coordinate system. The spins of the nuclei of the body are excited into precession about the z axis by means of a radio frequency (RF) pulse and the decaying precession of the spins produces an NMR signal. The amplitude of the NMR signal is dependent, among other factors, on the number of precessing nuclei per volume within the imaged body termed the "spin density".
Magnetic gradient fields G.sub.x, G.sub.y, and G.sub.z are applied along the x, y and z axes by means of gradient coils driven by a gradient amplifier system, so as to impress position information onto the NMR Signals through phase and frequency encoding, as is understood in the art. A set of NMR signals may be "reconstructed" to produce an image along a slice through the body. Each set of NMR signals is comprised of many "views", a view being defined as one or more NMR signal acquisitions made under the same x and y gradients fields.
Frequently it is desired to measure certain bio-potential signals, most notably the electrocardiogram signal ("ECG"), during an MRI scan. Such measurements may be required to monitor the status of critically ill patients or to synchronize the acquisition of MRI data with certain physiological phenomenon such as the beating of the heart. Bio-potential signals are intrinsically of low power and hence susceptible to electrical interference. In an MRI system, this interference comes principally from the previously discussed gradient fields when they are switched on and off during the MRI scan.
In order to reduce the interference from such external sources it is customary to filter the bio-potential signal with a low pass filter. For an ECG signal, the principle energy is below 100 Hz and hence a low pass filter having a cut-off frequency in this neighborhood is chosen. Unfortunately, depending on the imaging sequence used, the switching of the gradient fields also may produce significant interference below 100 Hz, thus limiting the effectiveness of the filtering.
Alternatively, the MRI scan may be "gated" and the bio-potential signal processed only for periods occuring during the times that the gradients were not being switched. The principle drawback to this gating method is that the bio-potential signal is still not available for periods of gradient switching and "continuous" measurements, such as that of heart rate for ECG, are therefore difficult to determine. Further, for rapid imaging techniques such as those to produce moving pictures of biological activity ("cine techniques"), the .gradient activity is essentially continuous, providing no period for acquiring of the bio-potential during the MRI scan.